The proposed work examines intrinsic features of HIV-1 genetic recombination and how these are affected by alterations in genome sequence and organization. HIV-1 genetic recombination occurs frequently and contributes both to disease progression in individual patients and to the continuing evolution of AIDS virus pandemic strains. Many fundamental questions about HIV-l genetic recombination remain to be resolved and are addressed in the work proposed here. These questions include how varying extents of sequence homology affect recombination frequencies, whether or not possession of identical leader regions is important to genetic recombination, and to what extent sequences such as host RNAs that lack known H1V-l packaging signals can participate in genetic recombination. In the proposed experiments, a series of modified HIV-1 genomes is used to test how variation like that naturally observed among HTV strains affects the frequency and the nature of recombinogenic template switching during single cycles of HIV replication in cultured human cells. The specific aims are 1) to establish assays for HIV template switching between two segments of a single RNA and for forced recombination between two RNAs 2) to test the effects of varying extents of sequence homology on template switching rates, and 3) to explore the relationship between specific determinants in the RNAs' 5' ends and genetic recombination frequencies. The work will provide fundamental knowledge about copy choice recombination which will contribute to understanding the genetic potential of populations of several kinds of RNA viruses, and will provide information useful in assessing the safety of lentiviral vectors and potential attenuated HIV vaccine strains.